J. E. Marr — Cambrian and Silurian Roclis. 247 



The second fauna is described as possessing Trilobites which 

 contrast with those of the primordial group ; either the head, thorax, 

 and tail ai'e pretty nearly in equilibrium, as in Asaphus, or the 

 thorax is reduced, and the head and tail developed, as in lUcenus. 

 But Illmius possesses no segments to the pygidium. and this is the 

 case with the primordial Microdiscus and Agnostus, which also have a 

 reduced thorax. Lastly there are several Trilobites of the second 

 fauna, comparable with others of the primordial, such as Calymene 

 with Conocoryphe, and Harpes with Erinnys, and the tails of these 

 do not possess very many segments, and are quite disproportional to 

 the thorax in their size, and the number of segments. 



M. Barrande believes that the second fauna is also characterized 

 by fossils which have never been found in the preceding fauna. 

 These are chiefly Cephalopods and Lamellibranchs. But the 

 occurrence of an abundance of Cephalopods and Lamellibranchs in 

 the earlier beds of the second fauna, e.g. in the Orthoceras Limestone 

 of Sweden, and in the Tremadoc rocks of Eamsey Island, as shown 

 by Dr. Hicks (Q.J.G.S. Feb. 1873), would indicate that they must 

 have previously existed in the earlier fauna ; migration with certain 

 physical conditions, as kindly suggested to me by Dr. Hicks, will 

 account for their absence, so far as known, in the European area. 

 In short we may object to the first of M. Barrande's arguments, 

 that the primordial beds have not been yet studied over a sufficiently 

 large area for us to say definitely that they are without Cephalopods 

 or Lamellibranchs anywhere ; in fact the evidence goes to point to 

 the fact that they must have existed somewhere at this time. To 

 the second argument we may object that characters which are 

 hardly of generic value in the Trilobites ' can scarcely be of much 

 value as assisting the classification of large groups of beds, especially 

 when there are so many exceptions to the rules laid down. 



M. Barrande proceeds to point out that although in his opinion 

 these three faunas are distinct, there exist nevertheless beds of 

 passage between them in certain countries. Such he considers to 

 be the Tremadoc rocks of England. In these beds he recognizes 

 rare representatives of the primordial fauna, as Gonocoryphe (Cono- 

 ceplialites) and Olenus, whilst with these appear Trilobites more 

 characteristic of the second fauna, as Niobe and Psilocephaliis or 

 Illcenus. In a paper on Bohemia, already referred to, I pointed out 

 reasons for concluding that Band D. d. 1 /3. of M. Barrande repre- 

 sented the Tremadoc rocks of England, and that among the scanty 

 fauna obtained from it, there occurred the primordial Harpides, with 

 Ampliion, a Trilobite found in Barrande's second fauna. The beds 

 of Hof, in Bavaria, described by M. Barrande as containing forms 

 both of primordial and second faunas, also seem to be referable to 

 the Tremadoc rocks. The Tremadoc rocks of Shropshire (cf. 

 Callaway, Q.J.Gr.S. vol. xxxiii. p. G52) also have a fauna somewlaat 



^ Of the genus Phacops, the subgenus Chasinops has a large tail and many seg- 

 ments, the subgenera Fbacnps proper and Acaste have much smaller ones, and few 

 segments relativplv to the thorax. 



